Ultraviolet radiation is known to induce various changes in the skin. Dermatologically, active wavelengths are divided into long-wavelength ultraviolet rays of 400-320 nm, medium-wavelength ultraviolet rays of 320-290 nm and short-wavelength ultraviolet rays shorter than 290 nm, which are called "UV-A", "UV-B" and "UV-C" respectively.
The majority of ultraviolet radiation sources to which human are exposed is usually sun light. Ultraviolet radiation which reaches the ground are UV-A and UV-B, while UV-C is absorbed in the ozonosphere and practically does not reach the ground. Of ultraviolet radiation which reaches the ground, UV-B induces changes in the skin, for example, development of erythema and blisters, enhancement of the formation of melanin, and cause of pigmentation when irradiated to the skin beyond a certain energy level. On the other hand, UV-A has conventionally been considered not to give too much changes to the skin. In recent years, it has however been found by electron microscopes or some histochemical methods that the skin may be affected upon its exposure to UV-A. Especially, unlike UV-B, the energy of UV-A reaches as deep as the corium, whereby the elastic fibers in vessel walls and connective tissue are caused to undergo semi-chronic changes. These changes are believed to lead to the acceleration of skin aging. UV-A is also known to make the skin darker immediately after its irradiation (immediate pigmentation) and to enhance the deleterious effects of UV-B to the skin. Accordingly, UV-A is considered to serve as one of causes for the occurrence or aggravation of chloasma or freckles.
As readily understood from the foregoing, it is important to protect the skin not only from UV-B but also from UV-A for the prevention of accelerated aging of the skin and the avoidance of occurrence or aggravation of chloasma or freckles.
It is however not very long since researches started on the effects of UV-A to the skin. Under the circumstances, there are not known many materials which can effectively absorb UV-A when applied to the skin. For the time being, dibenzoylmethane derivatives and cinnamic acid derivatives are only known to have such effects. Most of them are however fat-soluble (see, DE-OS No. 27 28 241 and 27 28 243; and Japanese Patent Laid-Open Nos. 61641/1976, 46056/1977 and 197209/1982), and there are only handful water-soluble materials (see, Japanese Patent Laid-Open No. 59840/1982). If one wants to incorporate such UV-A absorbents to cosmetic formulations, various limitations are imposed on the properties of the base materials in such cosmetic formulations. There is thus an outstanding demand for the development of a UV-A absorbent which has broader applicability. Such a UV-A absorbent is supposed to satisfy the following conditions.
(1) To have the maximum absorption wavelength around a wavelength of 350 nm.
(2) To have sufficiently large molar extinction coefficient (.epsilon.) at the above wavelength.
(3) To absorb little visible light, namely, to have .epsilon..apprxeq.0 at wavelengths of 400 nm and longer because it is not desirable to color cosmetic formulations.
(4) To be stable to heat and light.
(5) To have no toxicity, irritation and any other harmful effects to the skin.
(6) To have excellent compatibility with the base materials of cosmetics.
(7) To be resistant to percutaneous absorption and less susceptible to removal due to perspiration when applied to the skin, in other words, to maintain its effects effectively for a long period of time.
(8) To be inexpensive.